1. Field of the Invention
The present invention relates to a method of regenerating spent etching or oxidizing media based upon hexavalent chromium as the oxidizing agent. More particularly, the present invention relates to an electrolytic technique for regenerating hexavalent chromium oxidizing solutions while substantially reducing the quantities of impurity metal ions in solution.
2. Description of the Prior Art
Oxidizing solutions based upon hexavalent chromium in aqueous sulfuric acid have been employed in cleaning and etching operations in the treatment of plastics for plating, in brass finishing, in printed circuit board etching, in anodizing and in other surface treatments. As the oxidizing solutions are used, Cr.sup.+6 is reduced to Cr.sup.+3 and the dissolved solids content increases while the effective acid concentration diminishes. However, after use of such cleaning solutions, the problem which remains is disposal of the spent etching solutions, i.e., solutions whose etching rate is less than that required for a given operation.
In the past, substantial quantities of chromium values have been lost through discard of the solutions. Moreover, the technology for the ultimate disposal of chromium wastes usually requires the reduction of remaining Cr.sup.+6 to Cr.sup.+3. The solution containing trivalent chromium is then treated with a base to precipitate chromium values and other extraneous metals present as the hydroxides. The solid sludge obtained has been used as a landfill material which represents a substantial waste of valuable chromium values.
The use of chromic acid-sulfuric acid etchants in the fashion described above poses significant problems for the recovery of secondary resources and for pollution control. It is significant that the entire U.S. production of primary chromium is achieved from imported ores. In fact, more than 30,000 tons of chromium are employed annually for the oxidizing treatment of various surfaces and for corrosion control measures. Because of minimal recovery technology and because of the practices of the finishing industry which utilize Cr.sup.+6 oxidizing solutions, substantial quantities of valuable secondary chromium materials are lost. Furthermore, waste chromium has long been recognized as a major pollution problem. In fact, the prevailing EPA regulations prevent the discharge of waste water containing more than 0.25 ppm Cr.sup.+6 into sewers by plants discharging more than 10,000 gal/day of aqueous chromium wastes. Still further, suitable landfill areas for chromium hydroxide sludge materials are becoming increasingly scarce, and the collection, treatment and disposal of chromium wastes by contractors is expensive.
Because of the problem involved in the disposal of waste chromium values, it would be desirable to have a method by which spent chromic acid-sulfuric acid etching solutions could be regenerated and recycled for reuse within a plant. The recycle of such regenerated etching solutions would be very helpful in reducing the volume of chromium-containing effluents, thus reducing the costs of waste treatment and disposal, and in addition, conserving chromium resources.
The constraints upon a viable in-plant regenerating and recycling process for these etchants are more severe than those applicable for simple metal recovery processes. These constraints include the fact that reagent additions are limited since reagent removal would then be necessary. Any increase in the volumes of Cr solutions or decrease in concentration of Cr in solutions would necessitate some type of evaporative recovery process.
Etchants used to prepare plastic for plating present a different problem. The high Cr.sup.+6 concentration used results in a very corrosive solution. Since many plastic etching operations now use countercurrent rinsing coupled with evaporative recovery to recover chromium values lost in the rinse, it would be desirable to oxidize Cr.sup.+3 and remove contaminant metals from the rinse. The rinse could then be concentrated and returned to the etching tank. This procedure would minimize contamination of the etchant.
Several techniques have been proposed for the regeneration of waste chromium containing solutions. In the method disclosed by Lancy et al., U.S. Pat. No. 3,764,503, a solution containing hexavalent chromium values and other metal ions is introduced into one dialysis compartment of an at least two-compartment electrolytic dialysis unit. The solution to be regenerated is introduced into the anode compartment where oxidation of metal ions occurs. The metal ions then diffuse through a cation permeable membrane into another region of the dialysis unit. This second region of the unit either contains the cathode or is separated from the cathode compartment by an anion permeable membrane. If the second region contains the cathode, the cathode is of a metal ion pick-up inhibiting or restricting type, while if the cathode is in a region adjacent the second region, cation permeation into the cathode chamber is prevented by the anion permeable membrane. The patent, contrary to the principal objective of the present invention, does not show an electrolytic technique of reoxidizing Cr.sup.+3 values in a spent Cr.sup.+6 -acid solution. In fact, it only shows H.sub.2 CrO.sub.4 -acid solutions containing such extraneous metals as copper. Moreover, unlike the process of the present invention, the Cu.sup.+2 ions generated are prevented from building up in the catholyte. Dendritic growth also occurs in the catholyte of the patent.
Tirrell, U.S. Pat. No. 3,761,369, shows a technique of reclaiming spent etching fluids containing chromium values. However, the technique disclosed is one which is performed in two stages, in which in the first stage remaining Cr.sup.+6 ions in solution are reduced to Cr.sup.+3 ions. Following the reduction of Cr.sup.+6 ions to Cr.sup.+3 ions, copper metal is plated from the solution onto the cathode, with the termination of copper plating indicated by the electrolytic generation of hydrogen. After completion of metal reduction, the solution containing Cr.sup.+3 values is transferred to the anode chamber where Cr.sup.+3 ions are reoxidized to Cr.sup.+6 ions. Thus, the system of Tirrell is less efficient than the present process because Cr values are first reduced and then reoxidized. Fujii, U.S. Pat. No. 3,948,738, shows a two stage electrolytic diaphragm process for the regeneration of exhausted chromium-plating solutions which is the same as the process of Tirrell in that it at least involves initial cathodic reduction of hexavalent chromium to trivalent chromium and then transfer of the cathodic solution to the anode of the electrolyte cell where Cr.sup.+3 is reoxidized to Cr.sup.+6. Accordingly, the Fujii process is as inefficient as the Tirrell process with respect to the present process for the reasons mentioned above.
Lancy, U.S. Pat. No. 3,481,851, discloses the use of electrodialysis for removal of metallic contaminants, such as copper and iron, from spent aqueous chromium plating solutions. The chromium solution is contained in the anolyte compartment of the electrodialysis cell, with an aqueous mineral acid solution in the catholyte compartment and with the two compartments being separated by a cationic permeable membrane. Electrolysis causes the contaminant cations to migrate from the anolyte compartment to the catholyte compartment, while simultaneously oxidizing a portion of the trivalent chromium present in the spent solution to the hexavalent state. It has been found, however, that the efficiency of oxidation of trivalent to hexavalent chromium in the process of Lancy is relatively low, particularly as applied to spent chromium etching solutions. Therefore, a need continues to exist for a method by which exhausted Cr.sup.+6 etching or oxidizing solutions can be efficiently regenerated to reusable and functioning etching solutions.